How to pass arguments into event listener function in flex/actionscript?

A function called by a listener can only have one argument, which is the event triggering it.

listener:Function — The listener function that processes the event.
This function must accept an Event
object as its only parameter and must
return nothing, as this example
shows:

function(evt:Event):void

Source

You can get around this by having the function called by the event call another function with the required arguments:

timer.addEventListener(TimerEvent.TIMER, _saveChat);
function _saveChat(e:TimerEvent):void
{
    saveChat(arg, arg, arg);
}

function saveChat(arg1:type, arg2:type, arg3:type):void
{
    // Your logic.
}

Another thing you can do create a custom event class that extends flash.events.Event and create properties that you need within.

package
{
    import flash.events.Event;

    public class CustomEvent extends Event
    {
        // Your custom event 'types'.
        public static const SAVE_CHAT:String = "saveChat";

        // Your custom properties.
        public var username:String;
        public var chatBoxText:String;

        // Constructor.
        public function CustomEvent(type:String, bubbles:Boolean=false, cancelable:Boolean=false):void
        {
            super(type, bubbles, cancelable);
        }
    }
}

Then you can dispatch this with properties defined:

timer.addEventListener(TimerEvent.TIMER, _saveChat);
function _saveChat(e:TimerEvent):void
{
    var evt:CustomEvent = new CustomEvent(CustomEvent.SAVE_CHAT);

    evt.username = "Marty";
    evt.chatBoxText = "Custom events are easy.";

    dispatchEvent(evt);
}

And listen for it:

addEventListener(CustomEvent.SAVE_CHAT, saveChat);
function saveChat(e:CustomEvent):void
{
    trace(e.username + ": " + e.chatBoxText);
    // Output: Marty: Custom events are easy.
}